In recent years, a digital still camera and a digital video camera have widely spread as home use apparatuses, and further a miniaturized design has been required to those small-sized imaging apparatuses. On this account, an image-taking lens to be mounted, particularly a zoom lens is required to be miniaturized by shortening its total length and horizontal depth. Further, an improvement in the lens performance is also required for such image-taking lens for a digital still camera in response to an increase of the number of pixels in such imaging device in addition to the miniaturized design.
It has been known, for example, that a so-called rear focus type zoom lens, where lens groups other than a first lens group provided at the most object side are moved to focus, is made possible to easily miniaturize a total lens system, and to obtain an imaging performance suitable for a solid-state imaging device having a larger number of pixels. As such zoom lens of rear focus type, a zoom lens having five lens groups is well known, wherein the zoom lens is configured to have a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, a fourth lens group having a positive refracting power, and a fifth lens group having a negative refracting power, from object side in this order, and a zooming operation is carried out by moving the second and fourth lens groups, and a focusing operation is carried out by moving the fourth lens group. For example, there is a zoom lens configured as above which satisfies following equations (1) to (3), wherein focal lengths of the third lens group and the fifth lens group are f3, and f5, an imaging magnification of the fifth lens group at a position where an object distance is at infinity is β5, a focal length of the second lens group is f2, and focal lengths in this total system at a wide-end and a tele-end are fw, and ft, respectively (for example, refer to Japanese Patent Publication No. 3015192 (paragraph number [0014] to [0037], FIG. 1)).0.8<|f5/f3|<2.1  (1)1.2<|β5|<1.6  (2)0.25<|f2/√{square root over (fw·ft)}|<0.37  (3)
Recently, it has been considered to delete a projected portion of lens system during taking images by folding an optical path in the middle from the first lens group to an image plane to shorten the length of the lens when assembled in the imaging apparatus, and by setting a movable direction of the lens during zooming operation in the up and down directions. For example, there is a zoom lens that has a first lens group having a positive refracting power, a second lens group having an negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power, from an object side in this order. The zoom lens is configured to include a lens construction having four lens groups that performs a zooming operation by moving the second and fourth lens groups, wherein the first lens group includes a first lens of a single lens having an negative refracting power, a prism for folding an optical path, and a second lens of a single lens having a positive refracting power, from the object side in this order (for example, refer to Japanese Patent Application Publication No. 2000-131610 (paragraph number [0010] to [0027], FIG. 1)).
In the mean time, in the zoom lens having the optical system where the optical path is folded by a prism, it is possible to realize further miniaturization and a slim model design by miniaturizing a size of the prism. However in the zoom lens disclosed in Japanese Patent Application Publication No. 2000-131610, there is a problem that if a diameter and a thickness of lenses included in the first lens group is made small, it is difficult to further miniaturize the prism because of the deterioration of the optical performance.
This invention is presented in consideration of above-mentioned problem, and it is an object of the present invention to propose a zoom lens of rear focus type in which a total lens system is able to be miniaturized by further miniaturizing the prism without deteriorating an optical performance.
Further, another object of the present invention is to propose an imaging apparatus employing a zoom lens of rear focus type in which a total lens system is able to be miniaturized by further miniaturizing the prism without deteriorating an optical performance.